1. Field of the Invention
The present invention relates generally to power supplies, and more specifically to a method and apparatus for controlling a switching power converter entirely from the primary side of the power converter by predictive sensing of magnetic flux in a magnetic element.
2. Background of the Invention
Electronic devices typically incorporate low voltage DC power supplies to operate internal circuitry by providing a constant output voltage from a wide variety of input sources. Switching power converters are in common use to provide a voltage regulated source of power, from battery, AC line and other sources such as automotive power systems.
Power converters operating from an AC line source (offline converters) typically require isolation between input and output in order to provide for the safety of users of electronic equipment in which the power supply is included or to which the power supply is connected. Transformer-coupled switching power converters are typically employed for this function. Regulation in a transformer-coupled power converter is typically provided by an isolated feedback path that couples a sensed representation of an output voltage from the output of the power converter to the primary side, where an input voltage (rectified line voltage for AC offline converters) is typically switched through a primary-side transformer winding by a pulse-width-modulator (PWM) controlled switch. The duty ratio of the switch is controlled in conformity with the sensed output voltage, providing regulation of the power converter output.
The isolated feedback signal provided from the secondary side of an offline converter is typically provided by an optoisolator or other circuit such as a signal transformer and chopper circuit. The feedback circuit typically raises the cost and size of a power converter significantly and also lowers reliability and long-term stability, as optocouplers change characteristics with age.
An alternative feedback circuit is used in flyback power converters in accordance with an embodiment of the present invention. A sense winding in the power transformer provides an indication of the secondary winding voltage during conduction of the secondary side rectifier, which is ideally equal to the forward drop of the rectifier added to the output voltage of the power converter. The voltage at the sense winding is equal to the secondary winding voltage multiplied by the turns ratio between the sense winding and the secondary winding. A primary power winding may be used as a sense winding, but due to the high voltages typically present at the power winding, deriving a feedback signal from the primary winding may raise the cost and complexity of the feedback circuit. An additional low voltage auxiliary winding that may also be used to provide power for the control and feedback circuits may therefore be employed. The above-described technique is known as “magnetic flux sensing” because the voltage present at the sense winding is generated by the magnetic flux linkage between the secondary winding and the sense winding.
Magnetic flux sensing lowers the cost of a power supply by reducing the number of components required, while still providing isolation between the secondary and primary sides of the converter. However, parasitic phenomena typically associated with magnetically coupled circuits cause error in the feedback signal that degrade voltage regulation performance. The above-mentioned parasitics include the DC resistance of windings and switching elements, equivalent series resistance (ESR) of filter capacitors, leakage inductance and non-linearity of the power transformer and the output rectifier.
Solutions have been provided in the prior art that reduce the effect of some of the above-listed parasitics. For example, adding coupled inductors in series with the windings or a leakage-spike blanking technique reduce the effect of leakage inductance in flyback voltage regulators. Other techniques such as adding dependence on the peak primary current (sensed switch current) to cancel the effect of the output load on sensed output voltage have been used. However, the on-resistance of switches typically vary greatly from device to device and over temperature and the winding resistances of both the primary and secondary winding also vary greatly over temperature. The equivalent series resistance (ESR) of the power converter output capacitors also varies greatly over temperature. All of the above parasitic phenomena reduce the accuracy of the above-described compensation scheme.
In a discontinuous conduction mode (DCM) flyback power converter, in which magnetic energy storage in the transformer is fully depleted every switching cycle, accuracy of magnetic flux sensing can be greatly improved by sensing the voltage at a constant small value of magnetization current while the secondary rectifier is still conducting. However, no prior art solution exists that provides a reliable and universal method that adapts to the values of the above-mentioned parasitic phenomena in order to accurately sense the voltage at the above-mentioned small constant magnetization current point in DCM power converters.
Therefore, it would be desirable to provide a method and apparatus for controlling a power converter output entirely from the primary, so that isolation bridging is not required and having improved immunity from the effects of parasitic phenomena on the accuracy of the power converter output.